Metered dose inhalers are well known in medicine for the treatment of, or alleviation of the effects of, respiratory complaints, for example asthma. The medicament-containing liquid is held under pressure in a container which is mounted, usually removably, in an inhalation device. The aerosol container has a body portion, and a discharge stem movable with respect to the body portion from an inoperative position in which discharge is prevented to an operative position in which discharge takes place. To operate the inhalation device the patient needs to cause relative movement to take place between the body portion of the aerosol container and the discharge stem, for example by pressing on the closed end of the body portion, and this must be coordinated with inhalation on the part of the patient. Unfortunately, many patients needing this type of treatment are unable to coordinate their breathing with manual actuation of the device.
To try to overcome this problem devices have been proposed in which the act of inhalation itself causes the device to deliver a dose of medicament. Such devices are known as breath-actuated devices.
In designing a breath-actuated inhalation device there are a number of requirements which must be met but which are difficult to reconcile with one another. Thus, a substantial force is needed to move the body portion and discharge stem with respect to one another, but only a small force can be produced by inhalation, particularly in the case of the patients most likely to be using the device, which are persons having breathing difficulties. It has been proposed to overcome this problem by providing the inhalation device with an auxiliary energy store, for example in the form of a spring, the spring tending to cause the device to discharge a dose and being restrained from doing so by some form of restraining mechanism. The small force produced by inhalation is then used to release the restraining mechanism and permit the spring to operate the device.
However, this in turn gives rise to a further problem, which is that of maintaining the stability of the device. If the restraining mechanism can be released by the small force generated through inhalation then there is a risk that it may also be released accidentally when inhalation is not taking place. Conversely, if the restraining mechanism is held in place sufficiently securely to prevent accidental release, the force which a patient can generate by inhalation may be insufficient to release it.